Utilization of heat from condensate in indirectly steam heated multistage evaporators



June 1 1957 c. J. LOCKMAN ETAL 2,796,120

UTILIZATION OF HEAT FROM CONDENSATE IN INDIRECTLY STEAM HEATEDMULTISTAGE EVAPORATORS 3 Sheets-Sheet 1 June 18, 1957 c. J. LOCKMAN ETAL2,796,120

vUTILIZATION OF HEAT FROM CONDENSATE IN INDIRECTLY STEAM HEATEDHULTISTAGE EVAPORATORS 3 Sheets-Sheet 2 Filed April 10. 1951 June 1957c. J. LOCKMAN ETAL 2,796,120

UTILIZATION OF HEAT FROM CONDENSATE' IN INDIRECTLY STEAM HEATEDMULTISTAGE EVAPORATORS 3 Sheets-Sheet 3 Filed April 10. 1951 UnitedStates Patent UTELIZATION 9F HEAT FROM CONDENSATE IN KNDTRECTLY STEAMHEATED MULTISTAG EVAPURATORS Carl Johan Lockman, Stockholm, and SverreJenssen, Osirarstrom, Sweden, assignors, by mesne assignments, toRosenblad Corporation, New York, N. Y., a corporation of New YorkApplication April 10, 1951, Serial No. 220,172

2 Claims. (Cl. 159-47) This invention relates to a method and means forutilizing heat from condensate in indirectly steam heated multistageevaporators.

In evaporators of this kind it has been customary to allow thecondensate produced from the heating medium in the heat exchangers ofpreceding stages to discharge into the heating spaces of the heatexchangers of succeeding stages so as to release flash vapor under theinfluence of the drop in pressure occurring from heat exchanger to heatexchanger and to obtain a correspondingly lower temperature. In thismanner the condensate is passed through the evaporator from the first tothe last stage so that from every succeeding stage the condensate formedtherein will be discharged together with the condensate received fromthe preceding stage so that the total amount of condensate produced inthe evaporator will be collected in and discharged from the last stageat the com paratively low temperature of the heating medium in thatstage. This condensate may in turn be utilized for preheating the liquidto be evaporated, provided, of course, that this liquid is of lowertemperature than the condensate, whereby the heat thus transmitted isreturned to the evaporator with the liquid. It is, however, clear thatowing to the low temperature of the condensate it is impossible, in thismanner, to preheat the liquid to any considerable degree as comparedwith the temperature levels of the preceding stages and thus thepossibility of recovering heat in that way remains comparativelylimited.

It should be noted that the expressions preceding and succeeding, firstand last, used here to indicate the relative positions of the differentstages, relate to the direction of flow of the heating medium from stageto stage if not otherwise expressly stated.

The main object of the invention is to provide anew method of operationby means of which the liquid to be evaporated may be preheated to atemperature at most approaching the evaporation temperature of one ofthe preceding stages, preferably the first stage, this preheating beingettected substantially by means of condensate only, collected from thatstage and from the succeeding stages.

In the new method, contrary to known methods above described, thecondensate is passed and collected from succeeding stages to precedingstages, whereby in every preceding stage the condensate supplied from asucceeding stage will be brought into direct contact with the vaporousheating medium so as to be heated to at most approximately thetemperature of the heating medium and then, together with the condensateproduced from the heating medium in that stage, passed to the nextpreceding stage. In that way the condensate will be passed, heated andcollected from stage to stage, preferably from the last to the firststage and ultimately discharged at a temperature close to that of theheating medium of the stage from which said condensate is discharged andutilized for heating the liquid to be evaporated, which in turn ispassed to the last mentioned stage.

Assuming that the collected condensate is discharged 2 J96, llZ'llPatented June 18, 1957 from the first stage practically at thetemperature of the heating medium therein, it will be possible to heatthe liquid to a temperature at most, close to the evaporationtemperature of said stage. Assuming also that the original temperatureconditions for the liquid are not altered in relation to the comparedprior art, it will be possible thereby to cool the collected condensateto a temperature lower than that of the condensate formed in the laststage, so that heat from condensate formed in the evaporator, not onlyin the first stage, but also in all succeeding stages, will betransmitted to the liquid during its heating. By supplying the heatedliquid to the first stage, this heat will be completely returned to theevaporation process.

Further features of the invention will be explained in connection withthe following description with reference to the accompanying drawings,in which:

Fig. 1 diagrammatically illustrates an embodiment of the invention asapplied to a four-stage evaporator,

Fig. 2 shows a modification of a detail of Fig. 1 in section on a largerscale,

Fig. 3 shows part of a modified embodiment of the same evaporator as inFig. 1 on a larger scale, partly in section,

Fig. 4 shows a further modification as applied to a three-stageevaporator of a special kind, and

Fig. 5 illustrate a modification of Fig. 3.

In Fig. 1 each of the four stages comprises a heat exchanger 1, 2, 3 and4, preferably of the conventional tubular type (see Fig. 3), a steamseparator 5, 6, 7 and 8, a steam trap 9, 10, 1'1 and 12 and a condensatecollector 13, 14, 15 and 16 respectively. Within each stage the heatexchanger is connected to the associated steam separator through anoutlet 17 for liquid and vapor, and to the associated condensatecollector through an outlet 18 for heating medium condensate while theseparator communicates with the associated steam trap through a pipe 19for liquid. Each heat exchanger is provided with an inlet 20 forvaporous heating medium and an inletZl for liquid to be evaporated. Thevapor spaces of the separators 5, 6 and 7 are each connected to theheating medium inlets 20 of the succeeding stage through pipes 22', 23and 24, respectively. The steam traps 9 and II are each-adapted todischarge liquid to the inlets 21 of the succeeding stages through apipe 25. The steam trap 12 is adapted to dischargeliquid through a pipe26 to a receptacle 27 which through a pipe 2%, a pump 29, a check valve30, a pipe 31, a preheater 32 and a pipe 33 communicates with the inlet.21 of the heat exchanger 1. Steam is supplied to the system through apipe 34 communicating with the inlet Ztl of the heat exchanger 1, andvapor is discharged from the system by being drawn ott lfrom separator8; through a pipe 35, for instance to a condenser, not shown. The liquorto be evaporated is suppliedto the system through a pipe 36communicating with the inlet 21 of heat exchanger 3 and discharged fromthe system by being drawn off from steam trap 10 through a pipe 37..

It will be clear to any person skilled in the art that so far. theevaporator is one of conventional type in which the pathof flow of theheating medium from stage to stage. will follow the order 1-2-3-4 whilstthe path of flow of the liquid to be evaporated. will follow the stagewise order 34l-2. It will also be clear that in the evaporation of socalled black liquor, if the evaporator is arranged to operate with itsthird stage at evaporation temperature close to the temperature of theliquid supplied, there will be no possibility of heating that liquid bymeans of condensate at the condensing temperature of the last stage andthus to recover heat from the condensate in this manner.

According to. the invention, therefore, the condensate produced in theevaporator 4 and discharged into the collector 16 is passed through pump43, valve 46, check valve and pipe 33 into the pipe 23 so as to mix withthe heating vapor entering the evaporator 3, and in this manner thecondensate is heated to a temperature at most close to the temperatureof the vapor. In evaporator 3 the heated condensate will be mixed withthe condensate from the condensing vapor in that evaporator and bothquantities of condensate will be collected in the collector 15. Thecondensate thus collected and having approximately the same temperatureas the heating vapor is then passed through pump 44, valve 47, checkvalve 51 and pipe 39 into pipe 22 to be treated in stage 2 in the samemanner as now described. The process is repeated once more by passingthe collected condensate from the collector 14 through pump 45, valve48, check valve 52 and pipe 40 into pipe 34 and the total amount ofcondensate formed in the entire evaporator will be collected in thecollector 13 at a temperature at most close to the temperature of theheating medium of the first stage. the collector 13 the total amount ofcondensate is passed through pipe 41 and the surface heat exchanger 32in heat exchange relation to the liquor which passes from the last tothe first stage and thereby becomes heated to a temperature at mostclose to the evaporation temperature of the first stage. The condensateis ultimately discharged from the evaporator through valve 49 and outlet42. Each valve 46, 47, 48 and 49 is controlled by.a float 53 in theassociated collector 16, 15, 14 and 13, respectively, so as to operatein accordance with the condensate level in the collector. In order tomake certain that the condensate passed from one stage into the heatingvapor of the preceding stage quickly assumes a temperature at most closeto that of the heating vapor, we prefer to introduce the condensate intothe vapor in a finely divided state so as to mix intimately with theheating vapor, for instance as illustrated in Fig. 2 showing pipes 38,39 and 40 connected to pipes 23, 22 and 34, respectively, andterminating in spray nozzles 54, of any suitable type.

Fig. 3 illustrates one example of a modified manner of mixing thecondensate and the heating vapor. In this case the pipes 38, 39 and 40terminate in the top space of the collectors 15, 14 and 13 respectively,in which they discharge the condensate against a baflie plate 55 thusproducing a shower or spray of condensate which by causing the heatingvapor collected in this space to condense will assume a temperature atmost close to that of the vapor.

It should be understood that Fig. 3 merely shows one of the heatexchangers 1, 2 or 3 of Fig. 1 together with associated steam separator,modified condensate collector and pipes and that the generalconstruction of the evaporator may be the same as in Fig. 1.

It is of course necessary that the outlet 18 for the condensate producedin the adjacent heat exchanger 1, 2 or 3 and the collector issufliciently large to ensure that heating vapor together with thecondensate will pass continuously into, and be collected in theassociated collector in an amount sufiicient for the condensing andheating purposes. The baflie plate 55 is shown as an example only, andit should be understood that other means may be provided to distributethe condensate to be heated. Thus, for instance, a spray nozzle of thekind shown in Fig. 2 may also be utilized in the embodiment shown inFig. 3.

Fig. 4 illustrates diagrammatically an embodiment of the invention inconnection with an evaporator for the evaporation of scale formingliquid such as sulphite waste liquor in which the paths of flow of theheat exchanging media through the heat exchanger of each stage may beinterchanged periodically by means of valves 191, 192, 193, 194 and 195,196, 197, 19.8, respectively, in a similar manner to that described inLockmans U. S. Patent No. 2,488,598, so that during each period or cyclethe vaporous heating medium will be passed into those pas- From sages ofthe heat exchanger through which the liquid to be evaporated was passedduring the preceding period or cycle and vice versa, whereby the heatingmedium supplied will be mixed with its condensate in recirculation asdescribed in Lockmans pending U. S. patent application No. 206,752,filed Ianuary 19, 1951, now U. S. Patout No. 2,7345 65 for the purposeof increasing the quantity of condensate flooding those heatingsurfaces, which during the preceding period were exposed to the liquid,whereby scale formed on such surfaces will be dissolved and removed.

Each of the three stages has a heat exchanger 101, 102 and 103,respectively, each provided with separate passages 104 and 105 foralternately passing heating vapor and liquid in heat exchange relation.Condensate collected in the collector pertaining to heat exchanger 103(stage 3) is recirculated through a pump 151, a pipe 152, afloat-controlled valve 153 controlled by a float 180 and a pipe 154 andis mixed with the heating vapor supplied to the same stage in the pipe203, for instance by means of a spray nozzle 54 according to Fig. 2. Inthe same way condensate from collector 160 is recirculated in stage 2through pump 161, pipe 162, fioat controlled valve 163 and pipe 164 intosupply pipe 202 for the heating vapor and in stage 1 from collector 170through pump 171, pipe 172 and pipe 173 into supply pipe 201 for theheating vapor. In the full open position of valve 153 the pressure inpipe 156 exceeds the pressure in pipe 152. Consequently, no condensatefrom pipe 152 can pass to pipe 156 but the check valve prevents thecondensate from flowing in the reverse direction. As condensate formedin heat exchanger 103 is collected in the collector 150 the float 180will raise and the passage through valve 153 will be throttled until thepressure in pipe 152 exceeds the pressure in pipe 156 to a sufficientdegree to cause an equal quantity of condensate as that newly formed inand discharged into the collector from the heat exchanger, to passthrough check valve 155, pipe 156 and pipe 164 into the supply pipe 202in which the condensate, together with the recirculated condensate,becomes mixed with and heated by the heating medium supplied to stage 2.The total amount of condensate passes through the heat exchanger 102 tothe collector 160. From this collector condensate newly formed in heatexchanger 102 together with the condensate from stage 3 is passedthrough check valve 165, pipe 166 and pipe 173 into supply pipe 201 tobe treated in stage 1 in the same way as the condensate passed to stage2 is treated in the latter stage so that the total amount of condensateformed in the entire evaporator will ultimately arrive at the collector170 at a temperature which at most is close to that of the heatingmedium in the first stage.

The preheating and recovery of heat may be elfected in the embodimentaccording to Fig. 4 in a similar manner as described with reference toFig. 1. Similar parts of Figures 1 and 4 have the same referencecharacters.

However, the method described in connection with Fig. 4 is modified incomparison with that described with reference to Fig. 1 in as much as inthe first mentioned case, the process involves the step of introducingpassedover condensate together with recirculated condensate into theheating medium in order partly to utilize, for mixing the condensatewith the heating medium, means which already are available in the systemfor quite another purpose, namely for dissolving the scale. Theimportant similarity between the exemplified modifications of the methodaccording to the invention is inherent in the fact that the passed-overcondensate is heated by the heating vapor from the preceding stagebefore it is passed to the next preceding stage. In connection with theevaporator illustrated in Fig. 4 the invention would also be quiteoperative if the condensate, instead of being passed over to the pipes164 and 173 respectively, were passed over to the collectors andrespectively, in the manner shown and described in connection with Fig.3.

Fig. 5 illustrates a modification of Fig. 4. In Fig. 5 a restrictedpassage or a manually controlled valve 205 is provided in the pipe 154instead of the valve 153 in Fig. 4. In Fig. 5 the float-controlled valve153 is provided in the pipe 152 and is adapted to open when the liquidlevel rises in the collector 150. The restricted passage or valve 205 isadjusted so as to permit the condensate from the collector 150 to passinto the pipe 203 in a suificient amount. Alternatively the restrictedpassage may consist of the spray nozzle 54, the discharge area of whichmay be dimensioned so as to provide sufficient flow resistance.

What we claim is:

1. A method of utilizing heat from heating medium condensate inpre-heating liquid to be evaporated in indirect heat exchange withcondensable vaporous heating medium in a succession of evaporationstages, whereby every succeeding stage is heated by flash vapor releasedfrom the liquid from the immediately preceding stage, comprising thesteps of mixing condensate from one stage in a finely divided state withthe vaporous heating medium passing to the immediately preceding stageso as to condense a sufficient quantity of said vaporous heating mediumto heat said condensate to a temperature at most approaching the sametemperature as said vaporous heating medium, then mixing said heatedcondensate with the heating medium condensate thus formed, and with thecondensate formed in the same preceding stage from the remaining part ofsaid vaporous heating medium in heat exchange with the liquid evaporatedwith the last mentioned stage, subsequently passing the condensate thuscollected in heat exchange relation to liquid to be evaporated whichliquid has a temperature lower than that of the condensing temperatureof the heating medium of the first mentioned stage so as to cool thecollected condensate and heat the liquid and supplying the heated liquidto said preceding stage.

2. A method of utilizing heat from heating medium condensate inpreheating liquid to be evaporated in indirect heat exchange withcondensable vaporous heating medium in a succession of more than twoevaporation stages, whereby every succeeding stage is heated by flashvapor released from liquid from the immediately preceding stage,comprising the steps of mixing condensate from one stage, preferably thelast stage of the succession, in a finely divided state with thevaporous heating medium passing to the immediately preceding stage so asto condense a sufficient part of said vaporous heating medium to heatsaid condensate to a temperature at most approaching the sametemperature as said vaporous heating medium, then mixing said heatedcondensate with the heating medium condensate thus formed and with thecondensate formed in the same preceding stage from the remaining part ofsaid vaporous heating medium in heat exchange with the liquid evaporatedthere, subsequently treating the condensate thus collected in saidpreceding stage in the same way in connection with the next precedingstage, repeating such treatment and collection of condensate from stageto stage, passing the condensate thus ultimately treated and collectedfrom one ultimate stage of the treatment stages in heat exchangerelation to liquid to be evaporated having a lower temperature than thecondensing temperature of the heating medium of the first mentionedstage, preferably the last stage in the succession, so as to cool theultimately collected condensate and heat the liquid and supplying theheated liquid to said ultimate stage.

References Cited in the file of this patent UNITED STATES PATENTS643,702 Waterhouse Feb. 20, 1900 713,297 Goss Nov. 11, 1902 1,582,067Moore Apr. 27, 1926 2,012,668 Jahn Aug. 27, 1935 2,193,483 Hinckley Mar.12, 1940 2,262,519 Talton Nov. 11, 1941 2,535,777 Bickley Dec. 26, 19502,562,495 Hulme July 31, 1951 2,570,212 Cross Oct. 9, 1951 FOREIGNPATENTS 71,712 Sweden Dec. 5, 1928

